1. Field of the Invention
The invention relates generally to methods and devices for providing a stress-relieving joint between a riser and the keel of a floating platform.
2. Description of the Related Art
Deep water floating platforms use risers to communicate production fluid from the sea floor to the floating production platform. Floating platforms have a portion that lies below the surface of the sea. For stability of the platform, it is desired that there be a very deep draft. The spar, for example, is a popular style of floating platform that has an elongated, cylindrical hull portion which, when deployed, extends downwardly a significant distance into the sea. The lowest portion of the submerged hull is referred to as the keel. Currents in the sea tend to move the floating platform laterally across the sea surface. Despite the presence of anchorages, the platform imparts bending stresses to the riser during lateral movement. Localized, or point, stresses are particularly problematic for risers.
One known joint arrangement for use with risers and floating vessels is described in U.S. Pat. No. 5,683,205 issued to Halkyard. Halkyard describes an arrangement wherein a joint means is positioned within a keel opening in the floating vessel to reduce the amount of stress upon a pipe passing through the keel opening. The joint means consists of a radially enlarged sleeve member with an elastomeric annulus at either end that is in contact with both the sleeve member and the pipe. Halkyard""s intent is to reduce stress upon the pipe that is imposed by lateral movement of the floating vessel upon the sea. In order to reduce stress, Halkyard contacts the pipe at two points with an elastomeric annulus, which is described as providing a resilient, somewhat yieldable connection. Unfortunately, Halkyard""s arrangement is problematic since it permits almost no angular movement of the pipe within the sleeve member. While point stresses upon the pipe are reduced, they are still significant. Further, the pipe is required to bend within the confines of the sleeve. This bending, together with the induced point stresses at either end of the sleeve, place significant strain on the pipe.
The present invention addresses the problems in the prior art.
Keel joint assemblies are described that permit a degree of rotational movement of a riser within the keel of a floating vessel. The assemblies of the present invention greatly reduce the amount of stress and strain that is placed upon the riser, as well. The present invention describes keel joint assemblies that provide a limiting joint between the riser and the keel opening that permits some angular rotation of the riser with respect to the floating vessel. Additionally, the limiting joint permits the riser to move upwardly and downwardly within the keel opening, but centralizes the riser with respect to the keel opening so that the riser cannot move horizontally with respect to the keel opening.
In described embodiments, the limiting joint is provided by a single annular joint that allows that riser to move angularly with respect to the can. In some embodiments, the keel joint assembly incorporates a cylindrical stiffening can that radially surrounds a portion of the riser and is disposed within the keel opening. In these embodiments, a flexible joint is provided between the can and the riser. Supports or guides may be used to retain the can within the keel opening.